Gas turbines manufactured by the assignee are a so-called "can annular" design where 10, 14 or 18 combustion chambers or cans are arranged in a circle about the axial centerline of the gas turbine. The combustion cans are isolated from one another, except for the cross-fire tube connections between adjacent cans. The name of these tubes implies their function, i.e., the crossing of flame from one can to the next during ignition. The current gas turbine design incorporates two cans with ignition devices (spark plugs), while the other cans are lighted by the flame passing through the cross-fire tubes from the adjoining lighted can. Further, in the current Dry Low NOx gas turbine manufactured by the assignee, the cross-fire tubes must also pass flame from the lighted to the unlighted premixing regions of the combustion cans during transfer from a premixed mode to a lean-lean mode. In the premixed mode, the region of the combustor connected by cross-fire tubes has no flame and is used for premixing the fuel and air, while in the lean-lean mode this same region has flame. The specific function of the cross-fire tubes, whether during ignition or re-light of the premixing zone, is simply to pass flame from adjoining combustion cans. This process generally occurs in a matter of seconds. At all other times in the gas turbine operation, the cross-fire tubes perform no specific function.
When the cross-fire tubes are not in use, they must resist the unwanted passage of either hot gases from combustion or unburned fuel in the premixing zone from adjoining cans. This continuous cross-flow is due to chamber-to-chamber pressure differences resulting from small geometrical differences among the combustion hardware; from unequal distribution of fuel to the individual chambers; and from area variations in the gas turbine first stage nozzle passages. Continuous cross-flow of hot gas can permanently damage the combustion liner or cross-fire tube due to heating of the metal to its melting point. Some cooling is provided to the liner and cross-fire tube to protect against this cross-flow, but it is not robust enough for protection at high levels of cross-flow. Passage of unburned fuel from one can to the next produces a situation in the receiving can where the additional fuel produces streaks of fuel through the combustor. Hot streaks produced by the burning of this additional fuel may cause local over-heating of combustion components, or a situation where in the premixed mode, flame travels upstream with the fuel streak and produces a flashback event. A flashback event is a premature and unwanted re-light of the premixing zone during premixed mode operation, which produces an order of magnitude increase in NOx emissions due to the momentary transfer out of the premixed mode.
Specific to operating a Dry Low NOx combustor in the premixed mode with oil fuel, is the requirement that oil not be ingested into the cross-fire tubes. Unless protection is provided by design, there is a high probability this event will occur since the ends of the cross-fire tubes are located adjacent to the fuel nozzles to allow for ignition cross-firing. Given sufficient amount of time, No. 2 fuel oil, which is commonly used in gas turbine operation, will auto-ignite at temperatures above 400 to 500 degrees F. The baseline operating temperature is above 600 degrees F. and if oil does indeed settle into the cross-fires tube, it will remain there until either auto-igniting or burning by the cross-flow of hot gases.
The cross-fire tube configuration prior to this invention was designed to address the first stated problem, i.e., cross-flow of hot gas and/or unburned fuel. However, computer fluid dynamic (CFD) modeling of the air purge flow shows ineffective blockage of cross-flow through the tube. In the conventional practice, purge flow is admitted into the tube at each end with four equally spaced, opposed holes drilled into the wall of the tube. Air jets produced by the purge flow entering the tube coalesce at the tube axial centerline, such that the purge air is directed in both longitudinal directions. It has been determined that the major resistance to cross-flow occurs along the tube centerline and decreases toward the tube wall. With this pattern of purge air flow, hot gases or unburned fuel can bypass the air purge jets along the tube wall through the regions out of line with the air jets themselves. Thus, a flow condition can exist where even though cooling flow exits both ends of the tube, there is a continuous flow of gases from one chamber to the next, depending on chamber-to-chamber pressure differences.